1. Field of the Invention
This invention relates to a super-high-strength line pipe excellent in low temperature toughness and having a tensile strength (TS) of at least 900 MPa. This line pipe can be employed widely as a line pipe for transporting a natural gas and a crude oil.
2. Description of the Related Art
The importance of pipelines has been increasing, in recent years, as a long-distance transportation method for crude oil and natural gas. At present, trunk line pipes for long-distance transportation have been designed on the basis of API (American Petroleum Institute) Standard xc3x9765, and the pipelines designed in this way are overwhelmingly dominant in practice. However, high-strength line pipes have been desired in order (1) to improve transportation efficiency by higher pressurization and (2) to improve on-site working efficiency by the reduction of the outer diameter and weight of line pipes. Line pipes up to xc3x9780 (a tensile strength of at least 620 MPa) have been put into practical application to this date, but the need for line pipes having a higher strength has become apparent. Research into production methods of super-high-strength line pipes have been made on the basis of the production technologies of the conventional xc3x9780 line pipes (e.g. NKK Technical Report, No. 138(1992), pp.24-31 and The 7th Offshore Mechanics and Arctic Engineering (1988), Volume V, pp.179-185), but these studies are believed to be capable of producing at most xc3x97100 (tensile strength of at least 760 MPa) line pipes. Research into the production of steel plates for super-high-strength line pipes exceeding xc3x97100 have been made already (PCT/JP96/00155, 00157). However, the conventional seam welding technology cannot be applied to such super-high-strength line pipes and, even though the steel plates can be produced, the steel pipes cannot be produced unless the problems encountered in combining the seam weld portions with the steel plate are solved. Super-high-strengthening of the pipe lines involves a large number of problems to be solved, such as the balance between the strength and the low temperature toughness, the toughness of the weld heat affected zone (HAZ), field weldability, softening of the joint, and so forth, and early development of a revolutionary super-high-strength line pipe (exceeding xc3x97100) solving all these problems has been desired.
The present invention provides a super-high-strength line pipe that is excellent in the balance of low temperature toughness, insures easy field welding and has a tensile strength of at least 900 MPa (exceeding xc3x97100 of the API Standard), and a production method thereof.
The inventors of the present invention have conductive intensive studies in search of the conditions that must be satisfied, by a steel material and a seam weld portion, to provide a super-high-strength steel pipe having a tensile strength of at least 900 MPa and excellent in both low temperature toughness and field weldability, and have invented a novel super-high-strength line pipe and a production method thereof.
The gist of the present invention lies in the following points.
(1) A super-high-strength line pipe excellent in low temperature toughness and characterized in that a tensile strength of the steel pipe at a base metal steel plate portion in a circumferential direction is from 900 to 1,000 MPa, and a mean tensile strength of a weld metal used for welding the seam portions is at least the tensile strength of the steel plate xe2x88x92100 MPa.
(2) A super-high-strength line pipe excellent in low temperature toughness according to the item (1), wherein the Ni content of the weld metal is higher by at least 1% than the Ni content of the steel plate.
(3) A super-high-strength line pipe excellent in low temperature toughness according to the item (1) or (2), wherein the steel plate is shaped into a pipe shape at a UandO step, and is expanded into a pipe after the seam portions are welded from the inner and outer surfaces thereof by submerged arc welding.
(4) A super-high-strength line pipe excellent in low temperature toughness according to any of the items (1) through (3), wherein the steel plate contains, as component thereof:
C: 0.04 to 0.10%.,
Si: not greater than 0.6%,
Mn: 1.7 to 2.5%,
P: not greater than 0.015%,
S: not greater than 0.003%,
Ni: 0.1 to 1.0%,
Mo: 0.15 to 0.60%,
Nb: 0.01 to 0.10%,
Ti: 0.005 to 0.030%, and
Al: not greater than 0.06%,
contains selectively at least one of the following elements:
B: not greater than 0.0020%,
N: 0.001 to not greater than 0.006%,
V: not greater than 0.10%,
Cu: not greater than 1.0%,
Cr: not greater than 0.8%,
Ca: not greater than 0.01%,
REM: not greater than 0.02%, and
Mg: not greater than 0.006%, and
the balance of iron and unavoidable impurities; and
wherein the weld metal contains:
C: 0.04 to 0.14%,
Si: 0.05 to 0.40%,
Mn: 1.2 to 2.2%,
P: not greater than 0.010%,
S: not greater than 0.010%,
Ni: 1.3 to 3.2%,.
Cr+Mo+V: 1.0 to 2.5%,
B: not greater than 0.005%, and
the balance of iron and unavoidable impurities; and wherein the Ni content of the weld metal is higher by at least 1% than the Ni content of the steel sheet.
(5) A method of producing a super-high strength line pipe excellent in low temperature toughness, comprising the steps of:
shaping a steel plate having a tensile strength of 900 to 1,100 MPa into a pipe shape at a UandO step;
welding seam portions from inner and outer surfaces thereof by submerged arc welding using a welding wire containing Fe as a principal component, 0.01 to 0.12% of C, not greater than 0.3% of Si, 1.2 to 2.4% of Mn, 4.0 to 8.5% of Ni and 3.0 to 5.0% of Cr+Mo+V, and a sintered flux or a fused flux; and
expanding the steel plate into a pipe.
(6) A method of producing a super-high-strength line pipe excellent in low temperature toughness, comprising the steps of:
shaping into a pipe shape at a UandO step a steel plate having a tensile strength of 900 to 1,100 MPa and containing, as components thereof:
C: 0.04 to 0.10%,
Si: not greater than 0.6%,
Mn: 1.7 to 2.5%,
P: not greater than 0.015%,
S: not greater than 0.003%,
Ni: 0.1 to 1.0%,
Mo: 0.15 to 0.60%,
Nb: 0.01 to 0.10%,
Ti: 0.005 to 0.030%, and
Al: not greater than 0.06%,
xe2x80x83containing selectively at least one of the following elements:
B: not greater than 0.0020%,
N: 0.001 to not greater than 0.006%,
V: not greater than 0.10%,
Cu: not greater than 1.0%,
Cr: not greater than 0.8%,
Ca: not greater than 0.01%,
REM: not greater than 0.02%, and
Mg: not greater than 0.006%, and
the balance of Fe and unavoidable impurities;
welding seam portions of the steel plate from inner and outer surfaces thereof by submerged arc welding using a welding wire containing Fe as a principal component, 0.01 to 0.12% of C, not greater than 0.3% of Si, 1.2 to 2.4% of Mn, 4.0 to 8.5% of Ni and 3.0 to 5.0% of Cr+Mo+V, and a sintered flux or a fused flux; and
expanding the steel plate into a pipe.
(7) A method of producing a super-high-strength line pipe excellent in low temperature toughness according to the items (5) and (6), wherein the tensile strength of the weld metal of inner surface welding before pipe expansion is from 200 MPa below to the same as the tensile strength of the steel plate.